In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a few layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages.
Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.
EP 1 624 451 discloses a holographic storage system with a coaxial arrangement, where a plurality of reference beams are arranged around the object beam. According to this solution the object beam and the reference beams are coupled in and out at the object plane and the image plane, respectively. This arrangement is a so-called split aperture arrangement, because the aperture of the Fourier objective is split into an object part and a reference part. The arrangement has the advantage that the holographic material is near the Fourier plane, where the data density is the highest. Also, the overlap between the object beam and the reference beams is good near the Fourier plane. However, half of the aperture of the Fourier objective is used for the reference beams. This means that the capacity of a single hologram of the split aperture system is only half of the capacity of a common aperture arrangement. In addition, a total overlap of the object beam and the reference beams occurs only in the Fourier plane. The overlap is only partial within a 100-200 μm thick layer in the holographic storage medium. This value depends on the diameter of the hologram and the numerical aperture of the Fourier objective. Starting at a distance of about 200-400 μm from the Fourier plane there is no overlap at all. This limits the maximum useful thickness of the holographic material.
In WO2006/003077 a 12 f reflection type coaxial holographic storage arrangement with three confocally arranged Fourier planes is shown. In this arrangement the object beam and the reference beams are coupled in and out at the first and third Fourier planes, respectively. The reference beams are small spots in these planes. More precisely, they form diffraction patterns, similar to the Airy pattern. This arrangement is a so-called common aperture arrangement, because at the object plane and the image plane the object beam and the reference beams fill out the same area of the aperture. The beams fill out the entire aperture of the objectives. The disclosed arrangement allows to apply shift multiplexing, reference scanning multiplexing, phase coded multiplexing, or a combination of these multiplexing schemes. The reference beams are a pair (or pairs of) half cone shaped beams.
EP 1 912 212 discloses a holographic storage system with a coaxial arrangement of one or more reference beams and an object beam or a coaxial arrangement of one or more reference beams and a reconstructed object beam. The foci of the one or more reference beams are shifted relative to the focus of the object beam within the focal plane of the object beam or the reconstructed object beam.
EP 1 837 871 discloses a holographic storage medium with a holographic layer and a mirror layer. According to one embodiment the mirror layer has one or more reflective areas for reflecting one or more reference beams, whereas the remaining area of the mirror layer is transparent or absorptive.
In the known holographic storage systems the object beam and the reference beams do not fully overlap. To achieve a better overlap, the holographic material needs to be placed far from the Fourier plane. As a consequence, the single hologram data density in the holographic material is low.